1. Field of the Invention
The invention relates to internally toothed gear pumps for producing high pressure and, more particularly, the invention relates to an internally toothed gear pump having at least two essentially identical internal gears and a common pinion which meshes with the internal gears.
2. Description of Related Technology
An internal gear pump is known from DE 41 04 397 A1, published Sep. 12, 1991 (and counterpart UK Patent Publication No. 2,242,233, published Sep. 25, 1991). Such a pump includes two essentially identical internal gears and a common pinion which meshes with the internal gears, all of which are rotatably disposed in a common housing, the axial dimension of which is substantially equal to the width of the gear and pinion teeth. The pump includes a suction connection and a pressure connection. The internal gears have radial through-bores for pumping a medium therethrough.
In general, internally toothed gear pumps have a gear with internal teeth, which mesh with a pinion having a smaller number of outer teeth in driving connection with the internal teeth of the gear. Typically, each tooth of such a pump is relatively narrow in relation to the diameter of the pinion and the internal gear, respectively. Therefore, the volumetric flow conveyed by the pump is limited due to the construction thereof since the volumetric flow is determined by the height and width of the teeth.
In order to increase the volumetric flow in an internal gear pump, it is already known that the pump can be equipped with two (and, if desired, more) internal gears. Because the simple widening of an internal gear tooth is limited because of manufacturing and technological limitations, an increase of the volume flow can only be achieved by arranging two (and, if desired, more) internal gears coaxially, which then mesh together with a one-piece pinion. An internal gear pump of this type is disclosed as a variation of the embodiment disclosed in DE 41 04 397 A1.
The operation of this type of internal gear pump is such that the internal gears driven together by the pinion should cooperate synergistically as if only a single internal gear was present. However, in practice, when a pair of internal gears are used in an internal gear pump, the internal gears are pushed apart hydraulically at their axial contact surfaces. Moreover, it has also been observed that a pair of forces is generated at the internal gears which slants the gears relative to one another, i.e. tilting them. The reason for these inadequacies, which particularly promote wear, lies in the type and manner of hydrostatic relief of the internal gear pairs. Hydrostatic relief is defined herein as a pressure differential occurring between the inner and outer sides of an internal gear. It has been found that the hydrostatic pressure at the outer gear surface is greater than the hydrostatic pressure at the inner, toothed surface of the gear.